This invention relates to circuits for providing electromagnetic deflection of an electron-beam, and in particular, to circuits in which steady state sinusoidal current is employed to cause a zig-zag deflection.
A sinusoidal current for driving a horizontal deflection winding has been proposed. One advantage of using a sinusoidal current rather than the conventional ramp current lies in the fact that a lower power dissipation is achieved by a deflection circuit utilizing the sinusoidal current drive than in a comparable fast flyback type. This is so because in a system using sinusoidal deflection, the current flowing through the winding is primarily of the fundamental frequency of the driving current; whereas in a fast flyback the harmonics contents are higher, and the higher the harmonics contents of the deflection current, the higher the power dissipated in the ferrite of the winding and the losses due to the skin effect.
One feature of the invention is obtaining a sinusoidal-like current for driving deflection circuits of a picture tube by using a capacitively coupled symmetrical square wave voltage generator. The winding current provided by this square wave generator circuit may be characterized, however, as being a generally sinusoidal-periodic current having a period including the forward and return sweep durations in that the current in the fundamental, or first order frequency, is substantially stronger than the current in the harmonics. In some prior art circuitry the excitation voltage of the deflection circuit is not a capacitively coupled symmetrical square wave voltage generator, but a sine-wave generator providing a sine-wave voltage at its output stage. The output stage in the prior art operates as a linear amplifier; therefore, it requires more complicated circuit elements and it is less efficient for delivering power than the power stage in the square wave voltage generator of the invention which operates in switching mode. When a switch is closed, a DC voltage provided to the output stage is delivered to a load. This causes a small voltage drop on the switch impedance. Therefore, the power dissipated is small. Because the transition times are short in relation to the entire period of the square wave cycle, the energy dissipated during the transition time is small. In contrast, the output stage of a linear amplifier dissipates power during most of the cycle; therefore, its efficiency is lower than that of the output stage of the circuit of the invention.
In other prior art circuits, a pulse of current is provided during a short portion of the deflection cycle to a tank circuit which includes the inductance of the windings. The shortness of the pulse of current tends to result in undue stresses in the circuit components. In contrast, in the circuit of the invention, the energy is delivered during the entire cycle. The short pulse current of the prior art is required to deliver high power during the short period it is provided. This creates higher harmonic content in the sinusoidal-like current in the winding than in the sinusoidal-like current of the circuit of the invention.
Another feature of the invention is that it is possible to select the voltage of the generator from a wide range of voltages to achieve a predetermined amplitude of the sinusoidal current by selecting the relative capacitance of certain capacitors of the circuit.
Yet another aspect of the invention is the incorporation of a feedback loop circuit that reduces the dependency of the deflection current amplitude on circuit parameters.